With continued progress in the Human Genome Project as well as the initiation of the Mouse/Rat Genome Projects, and the sequencing of the majority of the human cDNAs, the elucidation of gene function has become a major priority. High throughput screening methods are required in order to determine the functions of large numbers of genes in an efficient manner. Screening methods are also required for discovering novel gene-specific drugs. In the search for such drugs, it would be advantageous to be able to elucidate the interaction between specific chemical reagents and one or more genes in a high throughput format. This is particularly relevant with respect to cancer drugs.
A synthetic or synergistic lethality screening method has previously been described in yeast cells (1,2). The essence of this screen in yeast, is the ability to identify nonallelic and nonessential mutations that are lethal in combination with a nonessential mutation in a gene of interest (i. e. synthetic lethality). A wild-type copy of the gene of interest, on an episomal plasmid, is introduced into cells which are null for expression of this gene. Random chemical mutagenesis of the entire yeast genome within these cells may inactivate a gene which is synthetically lethal with the gene of interest. Under these conditions, retention of the plasmid, which is otherwise spontaneously lost, and expression of the gene of interest become essential for survival. Plasmid loss or retention is detected by changes in colony pigmentation, due to the presence on the plasmid of a wild-type gene whose product is essential for red pigment accumulation (3).
This genetic method is very powerful as it can reveal not only interactions between gene products with direct physical contacts, but also interactions along the same or parallel pathways.